calculated from these channels. The input parameters available include the summation of currents from multiple input
channels. For the summed currents of phase, 3I_0 and ground current, current from CTs with different ratios are adjusted to
a single ratio before the summation.
A mechanism called a "Source" configures the routing of input CT and VT channels to measurement sub-systems.
Sources, in the context of the UR family of relays, refer to the logical grouping of current and voltage signals such that one
Source contains all of the signals required to measure the load or fault in a particular power apparatus. A given Source may
contain all or some of the following signals: three-phase currents, single-phase ground current, three-phase voltages and
an auxiliary voltage from a single VT for checking for synchronism.
To illustrate the concept of Sources, as applied to current inputs only, consider the breaker-and-a-half scheme as illustrated
in the following figure. In this application, the current flows as shown by the labeled arrows. Some current flows through the
upper bus bar to some other location or power equipment, and some current flows into transformer winding 1. The current
into winding 1 of the power transformer is the phasor sum (or difference) of the currents in CT1 and CT2 (whether the sum
or difference is used, depends on the relative polarity of the CT connections). The same considerations apply to trans-
former winding 2. The protection elements need access to the net current for the protection of the transformer, but some
elements may need access to the individual currents from CT1 and CT2.
Figure 5–1: BREAKER-AND-A-HALF SCHEME
In conventional analog or electronic relays, the sum of the currents is obtained from an appropriate external connection of
all the CTs through which any portion of the current for the element being protected could flow. Auxiliary CTs are required to
perform ratio matching if the ratios of the primary CTs to be summed are not identical. In the UR platform, provisions have
been included for all the current signals to be brought to the UR device where grouping, ratio correction and summation are
applied internally via configuration settings.
A major advantage of using internal summation is that the individual currents are available to the protection device, as addi-
tional information to calculate a restraint current, for example, or to allow the provision of additional protection features that
operate on the individual currents such as breaker failure.
Given the flexibility of this approach, it becomes necessary to add configuration settings to the platform to allow the user to
select which sets of CT inputs will be added to form the net current into the protected device.
The internal grouping of current and voltage signals forms an internal Source. This Source can be given a specific name
through the settings, and becomes available to protection and metering elements in the UR platform. Individual names can
be given to each Source to help identify them more clearly for later use. For example, in the scheme shown in the
BREAKER-AND-A-HALF SCHEME above, the user would configure one Source to be the sum of CT1 and CT2 and could
name this Source as 'Wdg 1 Current'.
Once the Sources have been configured, the user has them available as selections for the choice of input signal for the pro-
tection elements and as metered quantities.
GE Power Management
L90 Line Differential Relay